As the wheels on a locomotive wear down due to friction between the wheels and the railroad track, it is necessary to repeatedly calibrate the speedometer so that an accurate reading can be furnished to the operator or engineer. Additionally, in conventional cab signal systems there always exists the problem of calibrating the speedometer to the other elements of the cab signal system. That is to say, no discrepancy should appear between the two. Since many applications of cab signal systems incorporate wheel wear compensation, the calibration of the speedometer must be repeated every time the wheels are re-sized. A conventional method of performing this calibration is to manually adjust a potentiometer located inside the speedometer in accordance with a test signal.
The problem associated with the above-noted conventional calibration method is that it requires manual intervention, which is subject to error, and also necessitate the removal of the speedometer unit in order to adjust the potentiometer.
The present invention has overcome the above and other disadvantages by eliminating all manual intervention in the calibration of the speedometer. This not only removes the manpower requirements to re-adjust the speedometer, but also insures that the cab signal system and the speedometer always correspond to the same wheel size.
The present invention provides a speedometer calibration method and means which is vastly superior to another technique conventionally employed; one in which a microprocessor continually reads a speed probe and modifies the output signal to the speedometer drive to compensate for wheel size differences. This conventional technique results in error due to sampling rate and also consumes valuable processing time. In contrast, the present invention removes the processor from any wheel wear compensation tasks after the initial calibration, which is normally only performed on "power-up" of the system.
The present invention also provides many additional advantages which shall become apparent as described below.